An electron microscope uses electromagnetic lenses to focus on a specimen a primary electron beam emitted from an electron gun, then detect electrically charged secondary particles arising from the specimen, and acquire a magnified image of the specimen. A scanning electron microscope is an extended type of electron microscope with a function added to scan the primary electron beam across the specimen surface by use of electromagnetic or electrostatic deflectors placed above objective lenses.
In general imaging through a scanning electron microscope, a specimen is electrically grounded for its observation. Voltage may however be applied to the specimen to observe an image of the specimen. Recently, in particular, beam deceleration is coming to be most commonly used as a method of observing a specimen image. The deceleration method is a technique used to observe an image of a specimen by applying a negative voltage of nearly several hundreds of kilovolts to several kilovolts as a decelerating voltage to the specimen and decelerating a primary electron beam immediately in front of the specimen.
In the deceleration method, if an accelerating voltage applied from an electron gun to accelerate the primary electron beam is expressed as Vacc, and the decelerating voltage applied to the specimen is expressed as Vr, an irradiation voltage (also referred to as landing energy) Vi obtained when the primary electron beam reaches the specimen is expressed as Vi=Vacc−Vr. When the deceleration method is used, high image quality can be obtained as compared with that obtained when the deceleration method is not used (i.e., when the specimen is electrically grounded), even with the same irradiation. For example, although the irradiation voltage obtained at Vacc=1 kV and Vr=0.5 kV is the same as that obtained at Vacc=0.5 kV and Vr=0.0 kV, the former improves optical resolution meaning how clearly or sharply details of the specimen are imaged over the latter. In addition, the use of deceleration means enables the specimen image to be observed at a lower irradiation voltage (say, Vi=0.1 kV) than a minimum accelerating voltage (say, Vacc=0.5 kV) realizable with the electron gun. Thus, topographic observation of the uppermost surface of the specimen can be realized at high resolution. The observation with the deceleration method provides a variety of other advantages such as suppressing the build-up of electric charges in the specimen and reducing damage to the specimen.
Scanning electron microscopes can be classified into an out-lens type, a semi-in-lens type, and an in-lens type according to a particular layout relationship between objective lenses and a specimen. In the out-lens type of scanning electron microscope, the specimen is placed at a position completely distant from magnetic fields of the objective lenses, and in the in-lens type, the specimen is placed within the magnetic fields of the objective lenses. In the semi-in-lens type, which is somewhere between the out-lens type and the in-lens type, the specimen is placed at a location where the magnetic fields of the objective lenses partially leak. Of the three types of scanning electron microscopes, the in-lens type of scanning electron microscope capable of utilizing optical power of the objective lenses most efficiently is the most advantageous in that high-resolution images can be acquired.
In the in-lens type of scanning electron microscope that uses objective lenses (hereinafter, this microscope is referred to simply as in-lens SEM), a specimen needs to be placed between magnetic poles of the lenses in order to locate the specimen within the magnetic fields of the lenses. The specimen is therefore loaded into a position at an end of a special specimen holder and then inserted between the objective lenses to observe a magnified image of the specimen.
However, the deceleration method, an observation technique that involves applying a voltage of the same level (the same order of magnitude) as that of the accelerating voltage upon the primary electron beam, has the nature that the specimen needs to be loaded into position at the end of the special specimen holder and then inserted between magnetic poles of the objective lenses. This is likely to cause discharge and problems associated with safety. For this reason, the deceleration method has not been applied to in-lens SEM in the past.
On the other hand, techniques in which for other purposes a voltage, although not as high as the accelerating voltage, is applied to a specimen holder traditionally exist, primarily in the fields of transmission electron microscopes or scanning transmission electron microscopes. For example, Patent Document 1 below discloses an electron microscope in which, on a specimen holder with a plurality of specimens mounted thereupon, a memory is also mounted on the specimen holder to discriminate each of the specimens and an external power supply serving as a driving power supply for the memory is connected to the specimen holder via a cable.
According to Patent Document 1, a cable connection sensor that determines whether the cable is connected is disposed on a high-voltage lead-in connector as a measure to ensure safety associated with the application of the voltage. Thus, the application of the voltage to the specimen holder is inhibited if the cable connection sensor does not detect the connected state of the cable, or if a main body of the electron microscope fails to recognize the memory on the specimen holder.